The present invention relates, in general, to a dynamoelectric machine having a multi-part plug housing.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Electric equipment, for example dynamoelectric machines, are supplied with electrical energy via electric connecting lines, using plug-type connectors which also serve to transmit signal generator signals and sensor signals as well as performing functional grounding. Functional grounding is of considerable importance for interference-free functioning of the electric equipment. In this context, a differentiation is generally made between two types of grounding—protective grounding and functional grounding. While protective grounding serves to protect people and animals against electric shocks and is intended to provide protection only in the event of a fault, functional grounding constitutes a functional part of the usual operation of the electric equipment and is therefore essential for fault-free operation of the respective electric equipment.
Electric equipment, in particular in the case of industrial systems engineering, usually includes a metal housing which has to be provided with protective grounding for this reason and to comply with relevant DIN standards. Furthermore, functional grounding, which has the purpose of ensuring that the electric equipment is installed appropriately for EMC (electromagnet compatibility) is provided by the ground connection of the metal housing. Since the purpose of the protective ground is not aimed at the functioning of a piece of equipment but rather primarily serves for protecting persons, protective grounding alone is often not sufficient to ensure the electromagnetic compatibility of the electric equipment in addition to protecting persons. As a result, it is usually necessary to connect an additional functional ground. In this context, care should be taken to make sure that the functional ground and the protective ground are separated. There should normally be not connection of the protective ground to the functional ground.
The functional ground is generally not configured to perform protective conductor functions, and therefore when a protective conductor is connected to the functional grounding terminal the electrical safety of personnel is not ensured. A conductive connection between the protective ground and the functional ground is usually only provided in the vicinity of the actual ground. Operator protection has priority when performing grounding. Functional grounding of a piece of electric equipment is only given secondary importance.
Functional grounding has essentially the following objectives:                conduction away of interference currents and therefore improvement of the EMC,        definition of a necessary common reference potential in order to ensure operation of electric equipment.        
Protective conductors must be dimensioned to comply with the DIN-VDE specifications (DIN-VDE 0100 part 540 grounding, protective conductors, equipotential bonding conductors), and a correctly dimensioned protective conductor is suitable for conducting away electromagnetic interference only to a limited degree. Combined protective and functional grounding conductors should not only have the required cross section, but also a large surface area. In other words, the surface, in particular metallic surfaces, of the electric equipment should be connected in all cases to ground via a protective conductor connection in order to ensure adequate protection of persons. In the case of a single-part plug housing, a grounding pin of the plug is conductively connected to the plug housing, and the plug housing is in turn conductively connected to the electric equipment. A separate grounding line between the electric equipment and the plug is therefore not necessary.
However, in the case of multi-part plug housings whose parts are connected, for example, by a pivot joint, there is the risk of a conductive connection between these parts no longer being adequately ensured by the housing or via the pivot joint of the plug. This is the case, in particular, in the event of a fracture of the pivot joint.
In order to obtain grounding, a separate grounding line is led from the grounding pin of the plug to a conductive part of the motor housing in order always to be able to ensure adequate protection of persons. This grounding line is absolutely necessary if, for example owing to mechanical loading, the plug housing breaks at the pivot joint, and a conductive connection to the surface of the electric equipment is therefore interrupted. The motor in this case would continue to be operationally capable via the connecting lines but in the case of a fault, for example a short to ground of the winding, persons would no longer be protected.
The provision of a separate grounding line between the grounding pin of the plug and the electric equipment is disadvantageous because of the additional costs of such a high-temperature braided conductor line, an additional cable lug, the mounting of the line on the motor and plug, as well as a bore and a thread, for example in the motor end plate for attaching the cable lug. In particular in the case of motors which have a compact design and a low axle height, such mounting is particularly complicated.
It would therefore be desirable and advantageous to provide an improved dynamoelectric machine having a multi-part plug housing to obviate prior art shortcomings and to ensure sufficient protection of persons and, in particular in the case of dynamoelectric machines with a small axle height, to simplify assembly without adversely affecting operator protection.